U.S. patent number 6,301,421 [Application Number 09/318,554] was granted by the patent office on 2001-10-09 for photonic crystal fiber lasers and amplifiers for high power.
This patent grant is currently assigned to TRW Inc.. Invention is credited to Stephen J. Brosnan, Gerald W. Holleman, Michael G. Wickham.
United States Patent |
6,301,421 |
Wickham , et al. |
October 9, 2001 |
Photonic crystal fiber lasers and amplifiers for high power
Abstract
A photonic crystal fiber (10, 110) is provided for a
laser/amplifier system including a guiding structure comprising a
geometric array of axial passages (20, 120) formed along the length
of the fiber (10, 110). More particularly, the guiding structure
includes a central silica rod (14, 114) which is doped with a rare
earth element for providing optical gain to the laser/amplifier. A
plurality of second silica rods (16, 116) are disposed
circumferentially about the central rod (14, 114). Each of the
second rods (16, 116) includes an axial passage (20, 120) formed
therethrough along the length of the fiber (10, 110). A reflective
coating (22, 122) is deposited on an outboard surface of the array
of rods (12, 112) to confine pumped light (30, 130) therein. The
pumped light (30, 130) may be injected into the fiber (10, 110)
from the side by focusing it through small holes (24) in the
reflective coating (22) or by reflecting it off transverse Bragg
gratings (56) written into a fiber pigtail (50) coupled to the
fiber (110). The mode field diameter of the fiber (10, 110) is
controlled by properly selecting the diameter and spacing of the
passages (20, 120) in the second rods (16, 116).
Inventors: |
Wickham; Michael G. (Rancho
Palos Verdes, CA), Holleman; Gerald W. (Manhattan Beach,
CA), Brosnan; Stephen J. (San Pedro, CA) |
Assignee: |
TRW Inc. (Redondo Beach,
CA)
|
Family
ID: |
23238668 |
Appl.
No.: |
09/318,554 |
Filed: |
May 27, 1999 |
Current U.S.
Class: |
385/126; 372/6;
372/66; 372/70; 385/142 |
Current CPC
Class: |
G02B
6/02338 (20130101); G02B 6/02347 (20130101); H01S
3/06708 (20130101); G02B 6/02333 (20130101); H01S
3/0672 (20130101); H01S 3/06754 (20130101); H01S
3/094003 (20130101); H01S 3/1603 (20130101) |
Current International
Class: |
H01S
3/067 (20060101); H01S 3/06 (20060101); H01S
3/094 (20060101); H01S 3/16 (20060101); H01S
003/067 () |
Field of
Search: |
;385/123-127,141,142,144,146 ;372/6,69,70,66,92 ;359/341 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
5802236 |
September 1998 |
DiGiovanni et al. |
6058127 |
May 2000 |
Joannopoulos et al. |
6075915 |
June 2000 |
Koops et al. |
6097870 |
August 2000 |
Ranka et al. |
|
Other References
Properties of Photonic Crystal Fiber and the Effective Index Model;
Knight, Birks, and Russell; University of Bath, Bath, UK and
University of Southampton, Southampton, UK; Optical Society of
America; vol. 15, No. 3, Mar. 1998, pp. 748-752. .
Endlessly Single-Mode Photonic Crystal Fiber; Birks, Knight, and
Russell; University of Bath, Bath, UK and University of Southampton
, Southampton, UK, Optical Society of America; vol. 22, No. 13,
Jul. 1, 1997, pp. 961-963, Optics Letters..
|
Primary Examiner: Lee; John D.
Claims
What is claimed is:
1. An optical fiber for a laser/amplifier system comprising:
a first member doped with an optically pumpable material for
providing an optical gain to said laser/amplifier system;
a plurality of second members circumferentially disposed about said
first member, each of said second members including an axial
passage formed therethrough; and
a reflective coating deposited on a perimeter of said second
members;
wherein said reflective coating includes at least one opening
formed therein for receiving radiation therethrough.
2. The optical fiber of claim 1 wherein said optically pumpable
material further comprises a rare earth element.
3. The optical fiber of claim 2 wherein said rare earth element is
selected from a group including erbium, neodymium and
ytterbium.
4. The optical fiber of claim 1 wherein an index of refraction of
said second members matches an index of refraction of said first
member.
5. The optical fiber of claim 4 wherein said index of refraction of
said second members is controlled by adding an optically inactive
material thereto.
6. The optical fiber of claim 1 wherein said first and second
members further comprise silica.
7. A photonic crystal fiber comprising:
an array of rods including a central rod surrounded by a plurality
of second rods terminating in a perimeter of third rods;
said central rod being doped with an a rare earth element for
providing an optical gain;
each of said second and third rods including a hole formed therein
along a longitudinal axis thereof; and
a reflective coating deposited on an outboard surface of said third
rods;
wherein said reflective coating includes a plurality of openings
therein for receiving radiation therethrough.
8. The photonic crystal fiber of claim 7 further comprising a
second reflective coating deposited on both distal ends of said
array.
9. The photonic crystal fiber of claim 7 wherein said rare earth
element is selected from a group including erbium, neodymium and
ytterbium.
10. The photonic crystal fiber of claim 7 wherein an index of
refraction of said second and third rods matches an index of
refraction of said central rod.
11. The photonic crystal fiber of claim 7 wherein said rods further
comprise silica.
12. A method of forming a photonic crystal fiber for a
laser/amplifier system comprising the steps of:
providing a first member;
doping said first member with a rare earth element;
coupling a plurality of second members about said first member so
as to form an array, each of said second members including an axial
passage formed therethrough along a longitudinal axis thereof;
depositing a reflective coating on an outboard surface of said
array; and
forming at least one opening in said reflective coating for
receiving radiation therethrough.
Description
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention generally relates to laser and amplifier
systems and, more particularly, to laser and amplifier systems
employing fiber optics.
2. Discussion
Lasers and amplifiers employing fiber optics are known to be very
efficient. However, such systems are currently limited to
low-output powers since conventional single mode fibers can only
tolerate very small mode field diameters. As such, any attempt to
increase the power through the fiber causes high intensity at the
fiber output facet. This results in catastrophic damage.
Further, if an appropriate fiber was developed for tolerating such
increased power, a high level of pump power would be required. To
maintain the efficiency intrinsic with a fiber geometry, the pumped
radiation must propagate along the fiber axis. However,
conventional multi-mode high power pumps are difficult to focus on
the small cross-section at the end facet of a fiber.
In view of the foregoing, it would be desirable to provide an
efficient laser/amplifier fiber system capable of providing large
mode field diameters, as well as a mechanism to couple the required
pump power to the fiber.
SUMMARY OF THE INVENTION
The above and other objects are provided by a photonic crystal
fiber for a laser/amplifier system including a guiding structure
comprising a regular geometric (e.g. hexagonal) array of axial
passages formed along the length of the fiber. More particularly,
the guiding structure includes a central silica rod which is doped
with a rare earth element for providing optical gain to the
laser/amplifier. A plurality of second silica rods are disposed
circumferentially about the central rod. Each of the second rods
includes an axial passage formed therethrough along the length of
the fiber. The first and second rods are then drawn down to a
pre-selected diameter. This process transforms the first and second
rods into a sintered cluster in the form of a geometric array. A
reflective coating is deposited on an outboard surface of the array
to confine pumped light therein. The pumped light may be injected
into the fiber from the side by focusing it through small holes in
the reflective coating or by reflecting it off transverse Bragg
gratings written into a fiber pigtail coupled to the fiber. The
mode field diameter of the fiber is controlled by properly
selecting the diameter and spacing of the passages in the second
rods.
BRIEF DESCRIPTION OF THE DRAWINGS
In order to appreciate the manner in which the advantages and
objects of the invention are obtained, a more particular
description of the invention will be rendered by reference to
specific embodiments thereof which are illustrated in the appended
drawings. Understanding that these drawings only depict preferred
embodiments of the present invention and are not therefore to be
considered limiting in scope, the invention will be described and
explained with additional specificity and detail through the use of
the accompanying drawings in which:
FIG. 1 is a schematic perspective view of a photonic crystal fiber
laser/amplifier in accordance with a first embodiment of the
present invention;
FIG. 2 is a cross-sectional schematic view of a photonic crystal
fiber laser/amplifier according to the present invention; and
FIG. 3 is a schematic perspective view of a photonic crystal fiber
laser/amplifier according to a second embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed towards a photonic crystal fiber
for use in a laser or amplifier system. The photonic crystal fiber
of the present invention includes a rare earth element doped core
which yields large mode field diameters. Further, the photonic
crystal fiber of the present invention is efficiently coupled to a
high power pump by way of openings in a reflective coating or
transverse Bragg gratings written into a fiber pigtail so that high
power output may be provided.
Turning now to the drawing figures, FIG. 1 illustrates a photonic
crystal fiber according to a first embodiment of the present
invention. The photonic crystal fiber 10 includes a plurality of
members 12 aligned along a central axis of the photonic crystal
fiber 10. The members 12 are preferably formed of a transparent,
low-loss, damage resistant material such as silica. Further, the
members 12 are preferably elongated rods having an initial
cylindrical shape and macro-level size. However, after the members
12 are positioned adjacent one another they are heated until they
start to melt so that they begin to sinter together. Thereafter,
the cluster of sintered members 12 are subjected to repeated
drawing processes to reduce the diameter of the cluster. This
process continues the sintering of the members 12 so that the
boundaries between individual members 12 are somewhat lost.
Further, the drawing process causes the individual members 12 to
form a hexagonal array, but different geometries such as triangular
could be achieved by starting with a triangular cross section in
the members 12.
Referring momentarily to FIG. 2, the members 12 include a first or
central rod member 14 encircled by a plurality of second or
body-forming rod members 16. The outermost layer of second rod
members 16 is sometimes referred to hereinafter as third or
perimeter rod members 18. The central rod member 14 is preferably
doped with an optically pumpable material for providing an optical
gain to the photonic crystal fiber 10. The optically pumpable
material used for doping the central rod member 14 is preferably
formed of a rare earth element. For example, erbium, neodymium, and
ytterbium may be used. Also, the dopant preferably has a radial
profile selected to enhance the stability of low order modes
through the photonic crystal fiber 10.
The index of refraction of the second rod members 16 (including the
perimeter rod members 18) should match the index of refraction of
the doped central rod member 14. To accomplish this, it may be
necessary to add an optically inactive material to the second rod
members 16. For example, germanium may be used.
Each of the second rod members 16 includes an axial passage 20
formed therein along a longitudinal axis thereof and extending
along the length of the photonic crystal fiber 10. The diameter of
each passage 20 relative to the size of the second rod member it is
formed through is pre-selected such that upon drawing, a desired
mode field diameter of the photonic crystal fiber 10 is provided.
The spacing between the axial passages 20 may also be manipulated
to further control the mode field diameter.
Referring again to FIG. 1, a reflective coating 22 is deposited on
the outboard surface of the photonic crystal fiber 10 to confine
pump light therein. More particularly, the reflective coating 22 is
deposited on the outboard surface of the perimeter rod members 18.
Although the skilled artisan will appreciate that many reflective
materials are available for use as the reflective coating 22, it is
presently preferred to employ a metallic material such as silver or
aluminum. Alternatively, the reflective coating 22 may be formed by
encasing the fiber 10 in a sleeve of material having a different
index of refraction than the members 12.
A plurality of openings 24 are formed in the reflective coating 22
for enabling light to be injected therethrough and into the
interior of the photonic crystal fiber 10. To accomplish this, a
number of diode pump light sources 26, only one of which is
illustrated, are provided proximate the photonic crystal fiber 10.
A lens 28 is interposed between the diode pump light source 26 and
the photonic crystal fiber 10. The lens 28 focuses the multi-mode
radiation 30 from the diode pump light source 26 through the
adjacent opening 24. Thereafter, the radiation 30 is internally
reflected within the photonic crystal fiber 10 by the reflective
coating 22. The gain medium of the photonic crystal fiber 10, i.e.,
the core of the fiber 10 surrounded by the passages 20, is
energized when the multi-mode radiation 30 randomly reflects off of
the reflective coating 22 and traverses the doped central rod
member 14. Thereafter, the central rod member 14 guides the desired
mode along the length of the photonic crystal fiber 10.
When the fiber 10 is used as a laser, the mode is repeatedly
reflected off of a coating 32 disposed on both ends of the fiber 10
that is highly reflective at the laser and pump wavelengths. After
the mode gains sufficient power, it leaks through one of the
reflective coatings 32 in a steady stream. To accomplish this, it
is preferred to form one of the reflective coatings 32 with less
than 100% reflectivity at the laser wavelength.
Alternatively, when the fiber 10 is employed as an amplifier, both
ends of the fiber 10 are covered with a coating 32 which is
anti-reflective at the amplifier wavelength. As such, the mode is
allowed to propagate therethrough after a single pass. Although any
number of coating materials may be used for either the highly
reflective or anti-reflective coating 32, it is presently preferred
to use a dielectric material.
Turning now to FIG. 3, a second embodiment of the photonic crystal
fiber laser/amplifier of the present invention is illustrated. In
this embodiment, the same reference numerals as used in the first
embodiment are employed to identify similar elements in the second
embodiment but being increased by 100. The photonic crystal fiber
110 is identical to the photonic crystal fiber 10 of the first
embodiment with the exception that the openings 24 in the
reflective coating are omitted. As such, a second power pump
arrangement is employed for injecting radiation 130 into the
interior thereof.
To accomplish the foregoing, a fiber pigtail 50 is coupled to the
photonic crystal fiber 110. More particularly, the fiber pigtail 50
is coupled to the fiber 110 adjacent the central rod member 114.
When the photonic crystal fiber 110 is employed as an amplifier,
the pigtail 50 includes a core 52 wherein a signal 54 may be
injected. Radiation 130 from the diode pump light sources 126 is
focused by lenses 28 to impinge upon the Bragg gratings 56. The
radiation 130 is reflected therefrom to the gain medium, i.e., the
central rod member 114, of the photonic crystal fiber 110.
When the photonic crystal fiber 110 is employed as a laser,
dielectric coatings 58 and 59 are deposited at the ends of the
photonic crystal fiber 110. The dielectric coating 58 is highly
reflective at the laser wavelength and minimally reflective at the
pump wavelength. The dielectric coating 59 is optimally reflective
at the laser wavelength and highly reflective at the pump
wavelength. On the other hand, when the photonic crystal fiber 110
is employed as an amplifier, the dielectric coating 58 is not
present and dielectric coating 59 is anti-reflective at the
amplifier wavelength. In either case, the central rod member 114
guides the desired mode along the length of the photonic crystal
fiber 110 until it is exited therefrom as described above.
The precise periodic nature of the photonic crystal fiber of the
present invention enables a single mode fiber to be produced with
large mode field diameters. The large mode field diameters permit
the output power of the fiber to be substantially increased. Since
the photonic crystal fiber of the present invention can be side
pumped, the pump power can be distributed over a large area.
Further, the reflective surface of the photonic crystal fiber
confines the pump power therein so that it can be efficiently
absorbed by the doped core. Although the photonic crystal fiber of
the present invention will find usefulness in a myriad of
applications, it is particularly well suited for systems that
require high power, defraction-limited lasers in compact
configurations that are insensitive to misalignment.
Those skilled in the art can now appreciate from the foregoing
description that the broad teachings of the present invention can
be implemented in a variety of forms. For example, the fiber may be
made more rugged by applying a cladding or buffer layer of soft
polymer to the outside of the reflective surface. Therefore, while
this invention has been described in connection with particular
examples thereof, the true scope of the invention should not be so
limited since other modifications will become apparent to the
skilled practitioner upon a study of the drawings, specification,
and following claims.
* * * * *